Apache Log4j Security Vulnerabilities

This page lists all the security vulnerabilities fixed in released versions of Apache Log4j 2. Each vulnerability is given a security impact rating by the Apache Logging security team. Note that this rating may vary from platform to platform. We also list the versions of Apache Log4j the flaw is known to affect, and where a flaw has not been verified list the version with a question mark.

Log4j 1.x has reached End of Life in 2015 and is no longer supported. Vulnerabilities reported after August 2015 against Log4j 1.x were not checked and will not be fixed. Users should upgrade to Log4j 2 to obtain security fixes.

Binary patches are never provided. If you need to apply a source code patch, use the building instructions for the Apache Log4j version that you are using. For Log4j 2 these can be found in BUILDING.md located in the root subdirectory of the source distribution.

If you need help on building or configuring Log4j or other help on following the instructions to mitigate the known vulnerabilities listed here, please subscribe to, and send your questions to the public Log4j Users mailing list.

If you have encountered an unlisted security vulnerability or other unexpected behaviour that has security impact, or if the descriptions here are incomplete, please report them privately to the Log4j Security Team. Thank you!

Fixed in Log4j 2.17.1 (Java 8), 2.12.4 (Java 7) and 2.3.2 (Java 6)

CVE-2021-44832: Apache Log4j2 vulnerable to RCE via JDBC Appender when attacker controls configuration.

CVE-2021-44832 Remote Code Execution
Severity Moderate
Base CVSS Score 6.6 (AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H)
Versions Affected All versions from 2.0-alpha7 to 2.17.0, excluding 2.3.2 and 2.12.4

Description

Apache Log4j2 versions 2.0-beta7 through 2.17.0 (excluding security fix releases 2.3.2 and 2.12.4) are vulnerable to a remote code execution (RCE) attack where an attacker with permission to modify the logging configuration file can construct a malicious configuration using a JDBC Appender with a data source referencing a JNDI URI which can execute remote code. This issue is fixed by limiting JNDI data source names to the java protocol in Log4j2 versions 2.17.1, 2.12.4, and 2.3.2.

Mitigation

Log4j 1.x mitigation

Log4j 1.x is not impacted by this vulnerability.

Log4j 2.x mitigation

Upgrade to Log4j 2.3.2 (for Java 6), 2.12.4 (for Java 7), or 2.17.1 (for Java 8 and later).

In prior releases confirm that if the JDBC Appender is being used it is not configured to use any protocol other than Java.

Note that only the log4j-core JAR file is impacted by this vulnerability. Applications using only the log4j-api JAR file without the log4j-core JAR file are not impacted by this vulnerability.

Also note that Apache Log4j is the only Logging Services subproject affected by this vulnerability. Other projects like Log4net and Log4cxx are not impacted by this.

Release Details

From version 2.17.1, (and 2.12.4 and 2.3.2 for Java 7 and Java 6), the JDBC Appender will use JndiManager and will require the log4j2.enableJndiJdbc system property to contain a value of true for JNDI to be enabled.

The property to enable JNDI has been renamed from ‘log4j2.enableJndi’ to three separate properties: log4j2.enableJndiLookup, log4j2.enableJndiJms, and log4j2.enableJndiContextSelector.

JNDI functionality has been hardened in these versions: 2.3.1, 2.12.2, 2.12.3 or 2.17.0: from these versions onwards, support for the LDAP protocol has been removed and only the JAVA protocol is supported in JNDI connections.

Work in progress

The Log4j team will continue to actively update this page as more information becomes known.

Credit

No credit is being awarded for this issue.

References

Fixed in Log4j 2.17.0 (Java 8), 2.12.3 (Java 7) and 2.3.1 (Java 6)

CVE-2021-45105: Apache Log4j2 does not always protect from infinite recursion in lookup evaluation

CVE-2021-45105 Denial of Service
Severity Moderate
Base CVSS Score 5.9 (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H)
Versions Affected All versions from 2.0-beta9 to 2.16.0, excluding 2.12.3

Description

Apache Log4j2 versions 2.0-alpha1 through 2.16.0, excluding 2.12.3, did not protect from uncontrolled recursion from self-referential lookups. When the logging configuration uses a non-default Pattern Layout with a Context Lookup (for example, $${ctx:loginId}), attackers with control over Thread Context Map (MDC) input data can craft malicious input data that contains a recursive lookup, resulting in a StackOverflowError that will terminate the process. This is also known as a DOS (Denial of Service) attack.

Mitigation

Log4j 1.x mitigation

Log4j 1.x is not impacted by this vulnerability.

Log4j 2.x mitigation

Upgrade to Log4j 2.3.1 (for Java 6), 2.12.3 (for Java 7), or 2.17.0 (for Java 8 and later).

Alternatively, this infinite recursion issue can be mitigated in configuration:

  • In PatternLayout in the logging configuration, replace Context Lookups like ${ctx:loginId} or $${ctx:loginId} with Thread Context Map patterns (%X, %mdc, or %MDC).
  • Otherwise, in the configuration, remove references to Context Lookups like ${ctx:loginId} or $${ctx:loginId} where they originate from sources external to the application such as HTTP headers or user input.

Note that only the log4j-core JAR file is impacted by this vulnerability. Applications using only the log4j-api JAR file without the log4j-core JAR file are not impacted by this vulnerability.

Also note that Apache Log4j is the only Logging Services subproject affected by this vulnerability. Other projects like Log4net and Log4cxx are not impacted by this.

Release Details

From version 2.17.0, (and 2.12.3 and 2.3.1 for Java 7 and Java 6), only lookup strings in configuration are expanded recursively; in any other usage, only the top-level lookup is resolved, and any nested lookups are not resolved.

The property to enable JNDI has been renamed from ‘log4j2.enableJndi’ to three separate properties: ‘log4j2.enableJndiLookup’, ‘log4j2.enableJndiJms’, and ‘log4j2.enableJndiContextSelector’.

JNDI functionality has been hardened in these versions: 2.3.1, 2.12.2, 2.12.3 or 2.17.0: from these versions onwards, support for the LDAP protocol has been removed and only the JAVA protocol is supported in JNDI connections.

Work in progress

The Log4j team will continue to actively update this page as more information becomes known.

Credit

Independently discovered by Hideki Okamoto of Akamai Technologies, Guy Lederfein of Trend Micro Research working with Trend Micro’s Zero Day Initiative, and another anonymous vulnerability researcher.

References

Fixed in Log4j 2.16.0 (Java 8) and Log4j 2.12.2 (Java 7)

CVE-2021-45046: Apache Log4j2 Thread Context Lookup Pattern vulnerable to remote code execution in certain non-default configurations

CVE-2021-45046 Remote Code Execution
Severity Critical
Base CVSS Score 9.0 (AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H)
Versions Affected All versions from 2.0-beta9 to 2.15.0, excluding 2.12.2

Description

It was found that the fix to address CVE-2021-44228 in Apache Log4j 2.15.0 was incomplete in certain non-default configurations. When the logging configuration uses a non-default Pattern Layout with a Context Lookup (for example, $${ctx:loginId}), attackers with control over Thread Context Map (MDC) input data can craft malicious input data using a JNDI Lookup pattern, resulting in an information leak and remote code execution in some environments and local code execution in all environments; remote code execution has been demonstrated on MacOS, Fedora, Arch Linux, and Alpine Linux.

Mitigation

Log4j 1.x mitigation

Log4j 1.x is not impacted by this vulnerability.

Log4j 2.x mitigation

Implement one of the following mitigation techniques:

  • Upgrade to Log4j 2.3.1 (for Java 6), 2.12.3 (for Java 7), or 2.17.0 (for Java 8 and later).
  • Otherwise, in any release other than 2.16.0, you may remove the JndiLookup class from the classpath: zip -q -d log4j-core-*.jar org/apache/logging/log4j/core/lookup/JndiLookup.class

Users are advised not to enable JNDI in Log4j 2.16.0, since it still allows LDAP connections. If the JMS Appender is required, use one of these versions: 2.3.1, 2.12.2, 2.12.3 or 2.17.0: from these versions onwards, only the JAVA protocol is supported in JNDI connections.

Note that only the log4j-core JAR file is impacted by this vulnerability. Applications using only the log4j-api JAR file without the log4j-core JAR file are not impacted by this vulnerability.

Also note that Apache Log4j is the only Logging Services subproject affected by this vulnerability. Other projects like Log4net and Log4cxx are not impacted by this.

History

Severity is now Critical

The original severity of this CVE was rated as Moderate; since this CVE was published security experts found additional exploits against the Log4j 2.15.0 release, that could lead to information leaks, RCE (remote code execution) and LCE (local code execution) attacks.

Base CVSS Score changed from 3.7 (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L) to 9.0 (AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H).

The title of this CVE was changed from mentioning Denial of Service attacks to mentioning Remote Code Execution attacks.

Only Pattern Layouts with a Context Lookup (for example, $${ctx:loginId}) are vulnerable to this. This page previously incorrectly mentioned that Thread Context Map pattern (%X, %mdc, or %MDC) in the layout would also allow this vulnerability.

While Log4j 2.15.0 makes a best-effort attempt to restrict JNDI LDAP lookups to localhost by default, there are ways to bypass this and users should not rely on this.

Older (discredited) mitigation measures

This page previously mentioned other mitigation measures, but we discovered that these measures only limit exposure while leaving some attack vectors open.

Other insufficient mitigation measures are: setting system property log4j2.formatMsgNoLookups or environment variable LOG4J_FORMAT_MSG_NO_LOOKUPS to true for releases >= 2.10, or modifying the logging configuration to disable message lookups with %m{nolookups}, %msg{nolookups} or %message{nolookups} for releases >= 2.7 and <= 2.14.1.

The reason these measures are insufficient is that, in addition to the Thread Context attack vector mentioned above, there are still code paths in Log4j where message lookups could occur: known examples are applications that use Logger.printf("%s", userInput), or applications that use a custom message factory, where the resulting messages do not implement StringBuilderFormattable. There may be other attack vectors.

The safest thing to do is to upgrade Log4j to a safe version, or remove the JndiLookup class from the log4j-core jar.

Release Details

From version 2.16.0 (for Java 8), the message lookups feature has been completely removed. Lookups in configuration still work. Furthermore, Log4j now disables access to JNDI by default. JNDI lookups in configuration now need to be enabled explicitly. Users are advised not to enable JNDI in Log4j 2.16.0, since it still allows LDAP connections. If the JMS Appender is required, use one of these versions: 2.3.1, 2.12.2, 2.12.3 or 2.17.0: from these versions onwards, only the JAVA protocol is supported in JNDI connections.

From version 2.12.2 (for Java 7) and 2.3.1 (for Java 6), the message lookups feature has been completely removed. Lookups in configuration still work. Furthermore, Log4j now disables access to JNDI by default. JNDI lookups in configuration now need to be enabled explicitly. When enabled, JNDI will only support the JAVA protocol, support for the LDAP protocol has been removed.

From version 2.17.0 (for Java 8), support for the LDAP protocol has been removed and only the JAVA protocol is supported in JNDI connections.

From version 2.17.0 (for Java 8), 2.12.3 (for Java 7) and 2.3.1 (for Java 6), the property to enable JNDI has been renamed from ‘log4j2.enableJndi’ to three separate properties: ‘log4j2.enableJndiLookup’, ‘log4j2.enableJndiJms’, and ‘log4j2.enableJndiContextSelector’.

Work in progress

The Log4j team will continue to actively update this page as more information becomes known.

Credit

This issue was discovered by Kai Mindermann of iC Consult and separately by 4ra1n.

Additional vulnerability details discovered independently by Ash Fox of Google, Alvaro Muñoz and Tony Torralba from GitHub, Anthony Weems of Praetorian, and RyotaK (@ryotkak).

References

Fixed in Log4j 2.15.0 (Java 8)

CVE-2021-44228: Apache Log4j2 JNDI features do not protect against attacker controlled LDAP and other JNDI related endpoints.

CVE-2021-44228 Remote Code Execution
Severity Critical
Base CVSS Score 10.0 CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Versions Affected All versions from 2.0-beta9 to 2.14.1

Description

In Apache Log4j2 versions up to and including 2.14.1 (excluding security releases 2.3.1, 2.12.2 and 2.12.3), the JNDI features used in configurations, log messages, and parameters do not protect against attacker-controlled LDAP and other JNDI related endpoints. An attacker who can control log messages or log message parameters can execute arbitrary code loaded from LDAP servers when message lookup substitution is enabled.

Mitigation

Log4j 1.x mitigation

Log4j 1.x does not have Lookups so the risk is lower. Applications using Log4j 1.x are only vulnerable to this attack when they use JNDI in their configuration. A separate CVE (CVE-2021-4104) has been filed for this vulnerability. To mitigate: Audit your logging configuration to ensure it has no JMSAppender configured. Log4j 1.x configurations without JMSAppender are not impacted by this vulnerability.

Log4j 2.x mitigation

Implement one of the following mitigation techniques:

  • Upgrade to Log4j 2.3.1 (for Java 6), 2.12.3 (for Java 7), or 2.17.0 (for Java 8 and later).
  • Otherwise, in any release other than 2.16.0, you may remove the JndiLookup class from the classpath: zip -q -d log4j-core-*.jar org/apache/logging/log4j/core/lookup/JndiLookup.class

Note that only the log4j-core JAR file is impacted by this vulnerability. Applications using only the log4j-api JAR file without the log4j-core JAR file are not impacted by this vulnerability.

Also note that Apache Log4j is the only Logging Services subproject affected by this vulnerability. Other projects like Log4net and Log4cxx are not impacted by this.

History

Older (discredited) mitigation measures

This page previously mentioned other mitigation measures, but we discovered that these measures only limit exposure while leaving some attack vectors open.

The 2.15.0 release was found to still be vulnerable when the configuration has a Pattern Layout containing a Context Lookup (for example, $${ctx:loginId}). When an attacker can control Thread Context values, they may inject a JNDI Lookup pattern, which will be evaluated and result in a JNDI connection. While Log4j 2.15.0 makes a best-effort attempt to restrict JNDI connections to localhost by default, there are ways to bypass this and users should not rely on this.

A new CVE (CVE-2021-45046, see above) was raised for this.

Other insufficient mitigation measures are: setting system property log4j2.formatMsgNoLookups or environment variable LOG4J_FORMAT_MSG_NO_LOOKUPS to true for releases >= 2.10, or modifying the logging configuration to disable message lookups with %m{nolookups}, %msg{nolookups} or %message{nolookups} for releases >= 2.7 and <= 2.14.1.

The reason these measures are insufficient is that, in addition to the Thread Context attack vector mentioned above, there are still code paths in Log4j where message lookups could occur: known examples are applications that use Logger.printf("%s", userInput), or applications that use a custom message factory, where the resulting messages do not implement StringBuilderFormattable. There may be other attack vectors.

The safest thing to do is to upgrade Log4j to a safe version, or remove the JndiLookup class from the log4j-core jar.

Release Details

As of Log4j 2.15.0 the message lookups feature was disabled by default. Lookups in configuration still work. While Log4j 2.15.0 has an option to enable Lookups in this fashion, users are strongly discouraged from enabling it. A whitelisting mechanism was introduced for JNDI connections, allowing only localhost by default. The 2.15.0 release was found to have additional vulnerabilities and is not recommended.

From version 2.16.0 (for Java 8), the message lookups feature has been completely removed. Lookups in configuration still work. Furthermore, Log4j now disables access to JNDI by default. JNDI lookups in configuration now need to be enabled explicitly. Users are advised not to enable JNDI in Log4j 2.16.0, since it still allows LDAP connections. If the JMS Appender is required, use one of these versions: 2.3.1, 2.12.2, 2.12.3 or 2.17.0: from these versions onwards, only the JAVA protocol is supported in JNDI connections.

From version 2.12.2 (for Java 7) and 2.3.1 (for Java 6), the message lookups feature has been completely removed. Lookups in configuration still work. Furthermore, Log4j now disables access to JNDI by default. JNDI lookups in configuration now need to be enabled explicitly. When enabled, JNDI will only support the JAVA protocol, support for the LDAP protocol has been removed.

From version 2.17.0 (for Java 8), support for the LDAP protocol has been removed and only the JAVA protocol is supported in JNDI connections.

From version 2.17.0 (for Java 8), 2.12.3 (for Java 7) and 2.3.1 (for Java 6), the property to enable JNDI has been renamed from ‘log4j2.enableJndi’ to three separate properties: ‘log4j2.enableJndiLookup’, ‘log4j2.enableJndiJms’, and ‘log4j2.enableJndiContextSelector’.

Work in progress

The Log4j team will continue to actively update this page as more information becomes known.

Credit

This issue was discovered by Chen Zhaojun of Alibaba Cloud Security Team.

References

Fixed in Log4j 2.13.2 (Java 8) and 2.12.3 (Java 7)

CVE-2020-9488: Improper validation of certificate with host mismatch in Apache Log4j SMTP appender.

CVE-2020-9488
Severity Low
CVSS Base Score 3.7 (Low) CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Versions Affected All versions from 2.0-alpha1 to 2.13.1

Description

Improper validation of certificate with host mismatch in Log4j2 SMTP appender. This could allow an SMTPS connection to be intercepted by a man-in-the-middle attack which could leak any log messages sent through that appender.

The reported issue was caused by an error in SslConfiguration. Any element using SslConfiguration in the Log4j Configuration is also affected by this issue. This includes HttpAppender, SocketAppender, and SyslogAppender. Usages of SslConfiguration that are configured via system properties are not affected.

Mitigation

Users should upgrade to Apache Log4j 2.13.2 which fixed this issue in https://issues.apache.org/jira/browse/LOG4J2-2819 by making SSL settings configurable for SMTPS mail sessions. As a workaround for previous releases, users can set the mail.smtp.ssl.checkserveridentity system property to true to enable SMTPS hostname verification for all SMTPS mail sessions.

Credit

This issues was discovered by Peter Stöckli.

References

Fixed in Log4j 2.8.2 (Java 7)

CVE-2017-5645: Apache Log4j socket receiver deserialization vulnerability.

CVE-2017-5645
Severity Moderate
CVSS Base Score 7.5 (AV:N/AC:L/Au:N/C:P/I:P/A:P)
Versions Affected All versions from 2.0-alpha1 to 2.8.1

Description

When using the TCP socket server or UDP socket server to receive serialized log events from another application, a specially crafted binary payload can be sent that, when deserialized, can execute arbitrary code.

Mitigation

Java 7 and above users should migrate to version 2.8.2 or avoid using the socket server classes. Java 6 users should avoid using the TCP or UDP socket server classes, or they can manually backport the security fix commit from 2.8.2.

Credit

This issue was discovered by Marcio Almeida de Macedo of Red Team at Telstra

References

Summary of security impact levels for Apache Log4j

The Apache Log4j Security Team rates the impact of each security flaw that affects Log4j. We've chosen a rating scale quite similar to those used by other major vendors in order to be consistent. Basically the goal of the rating system is to answer the question “How worried should I be about this vulnerability?”.

Note that the rating chosen for each flaw is the worst possible case across all architectures. To determine the exact impact of a particular vulnerability on your own systems you will still need to read the security advisories to find out more about the flaw.

We use the following descriptions to decide on the impact rating to give each vulnerability:

Severity CVSS v3 Score Range
Critical 9.0 - 10.0
High 7.0 - 8.9
Moderate 4.0 - 6.9
Low 0.1 - 3.9

Critical

A vulnerability rated with a Critical impact is one which could potentially be exploited by a remote attacker to get Log4j to execute arbitrary code (either as the user the server is running as, or root). These are the sorts of vulnerabilities that could be exploited automatically by worms. Critical vulnerabilities score between 9.0 and 10.0 on the CVSS v3 calculator.

High

A vulnerability rated as High impact is one which could result in the compromise of data or availability of the server. For Log4j this includes issues that allow an easy remote denial of service (something that is out of proportion to the attack or with a lasting consequence), access to arbitrary files outside of the context root, or access to files that should be otherwise prevented by limits or authentication. High vulnerabilities score between 7.0 and 8.9 on the CVSS v3 calculator.

Moderate

A vulnerability is likely to be rated as Moderate if there is significant mitigation to make the issue less of an impact. This might be because the flaw does not affect likely configurations, or it is a configuration that isn't widely used. Moderate vulnerabilities score between 4.0 and 6.9 on the CVSS v3 calculator.

Low

All other security flaws are classed as a Low impact. This rating is used for issues that are believed to be extremely hard to exploit, or where an exploit gives minimal consequences. Low vulnerabilities score between 0.1 and 3.9 on the CVSS v3 calculator.

CVE creation process

Found security vulnerabilities are subject to voting (by means of lazy approval, preferably) in the private security mailing list before creating a CVE and populating its associated content. This procedure involves only the creation of CVEs and blocks neither (vulnerability) fixes, nor releases.